1. Field of the Invention
The present invention relates to a movement transforming device to be used for transforming rotational movement into linear movement or transforming linear movement into rotational movement, and relates to a power steering apparatus where the movement transforming device is used as a main component.
2. Description of the Background Art
An electric power steering apparatus, which is constituted so that an electric motor for steering assistance is driven according to a steering operation and a rotating force of this motor is transmitted to a steering mechanism so as to assist steering, is put to practical use. Most of such power steering apparatuses are constituted so that the motor for steering assistance is provided around a steering shaft (a rack shaft or the like in a rack-and-pinion steering mechanism) connected with tire wheels for steering (generally, right and left front tire wheels). A rotating force of the motor is directly transmitted to the steering shaft and thus the steering shaft is moved in the axial direction so that the steering is assisted.
In order to realize such a constitution, a movement transforming device for transforming rotation of the motor into movement of the steering shaft in the axial direction is required. As for a conventional device, as disclosed in Japanese Patent Application Laid-Open No. 61-191468 (1986), a movement transforming device utilizing a ball screw is widely used.
In the movement transforming device disclosed in the Japanese Patent Application Laid-Open No. 61-191468 (1986), a thread groove is formed on an outer circumference of the steering shaft, whereas a ball nut, which is fitted into the thread groove via a number of balls, is provided in a housing for supporting the steering shaft with movement of the ball nut in the axial direction being restricted. The rotating force of the motor for steering assistance is transmitted to the ball nut, and the steering shaft formed with the thread groove is moved in the axial direction by utilizing a screw movement of the thread groove due to the rotation.
Since such a conventional movement transforming device transforms the rotation of the ball nut into the movement of the steering shaft in the axial direction according to rolling movement of the balls intervening therebetween, high transmission efficiency is obtained. Moreover, since the apparatus including the motor for steering assistance can be structured compactly within a limited space around the steering shaft, the apparatus can meet a demand in reducing an installation space.
However, in the above-mentioned conventional ball-screw type movement transforming device, high accuracy is required for the thread groove formed on the outer circumference of the steering shaft, and thus there arises a problem that numerous processing steps are required. Moreover, a lot of labor is required for adjusting the fitting condition between the steering shaft and the ball nut via the balls, and thus there arises a problem that a number of assembling steps is increased.
In addition, since the plurality of balls, fitted into the thread groove, are circulated by a circulating mechanism provided relative to the ball nut, there arises a problem such that the structure of the ball nut becomes complicated. Further, according to the circulation of the balls in the circulating mechanism, generation of ball collision noises are not avoidable, and such noises would be heard by a driver as harsh grating sounds.
Under these circumstances, it is eager to realize a new compact movement transforming device which is capable of solving the aforementioned problems carried by the ball-screw type movement transforming device for transmitting the rotation of the motor for steering assistance to the steering shaft with high transmission efficiency equivalent to that of the ball-screw type movement transforming device.
As a movement transforming device which can respond to many movements, Japanese Patent Application Open No. 59-12898 (1984) discloses a movement transforming device, for example. The movement transforming device disclosed in Japanese Patent Application Open No. 59-12898 (1984) includes a moving shaft supported movably in the axial direction, a rotating cylinder which is supported rotatively on its axis and coaxially surrounds a periphery of the moving shaft, and a plurality of feed rings which are supported eccentrically to an inside of the rotating cylinder and whose inner surfaces are engaged with a circumferential surface of the moving shaft at one portion in the circumferential direction. As for the feed rings, rolling bearings, which have a plurality of rolling members such as balls and rollers between their inner rings and outer rings, are used where the inner rings having their inner diameter are sufficiently larger than an outer diameter of the moving shaft.
In addition, Japanese Patent Application Laid-Open No. 59-9351 (1984) discloses a similar movement transforming device in which a thread groove is formed on an outer circumferential surface of a moving shaft, and engaging projections provided around inner rings of feed rings are engaged with the thread groove so that their engagement is reinforced.
In these conventional movement transforming devices, when the rotating cylinder rotates on its axis, the feed rings held on the rotating cylinder rotate with the inner rings being engaged with the moving shaft, and the moving shaft is moved in the axial direction by an action of a component of the force in the axial direction of the engagement portion with the inner ring, and thus the rotation of the rotating cylinder is transformed into the axial movement of the moving shaft.
Since the rolling movement of the feed rings at this time is generated via the rolling members such as balls and rollers intervening between the inner rings and outer rings, the above-mentioned movement transformation displays the transmission efficiency which is substantially equivalent to that of the conventional ball-screw type movement transforming device. Moreover, since these rolling members are held between the inner rings and outer rings with the relative positions of the rings unchanged, collision of the rolling members does not occur, and thus quietness can be improved. Further, the rotating cylinder has a simple structure where the plurality of feed rings (bearings) are held to its inner side, and thus this device can achieve a greatly simple structure compared with the conventional ball-screw type movement transforming device.
However, even in the movement transforming device having the above-mentioned structure, it is indispensable for adjusting the engagement condition between the feed rings and the moving shaft to perform the satisfactory movement transforming operation.
An explanatory diagram of FIG. 1 shows the engagement condition between the feed rings and the moving shaft of the movement transforming device disclosed in Japanese Patent Application Open No. 59-12898 (1984).
As shown in FIG. 1, a plurality of feed rings 63 (only one is shown) are constituted so that a plurality of balls are intervened between inner rings 63a and outer rings 63b. The feed ring 63 is inserted into a mounting hole 60 which has a width corresponding to an outer diameter of the outer ring 63b and pierces through a predetermined position of a rotating cylinder 6. The feed ring 63 is constituted so that a block-shaped pressure member 61 which is inserted from one side of the mounting hole 60 is brought into contact with a half portion of the outer ring 63b. Here, the pressure members 61 are restricted collectively by cylindrical springs 62 mounted to the outside of the rotating cylinder 6 so as not to slip out of the mounting holes 60. In this state, the inner rings 63a of the feed rings 63 are engaged with the moving shaft 7 in inner sides of their contact positions with the pressure members 61.
In this conventional structure, the engagement condition between each of the plurality of feed rings 63 and moving shaft 7 is determined by the accuracy of a dimension ("X" in FIG. 1) between inner and outer surfaces of each of the pressure members 61. Therefore, when adjusting the engagement condition, it is required to adjust a dimension of the pressure members 61 which have a non-uniform section formed with the inner surface corresponding to an external shape of the feed rings 63 and the outer surface corresponding to an external shape of the rotating cylinders 6. As a result, there arises a problem such that numerous processing steps are required.
In addition, since the engaging strength between the feed rings 63 and moving shaft 7 depends on spring forces of the springs 62 elastically contacting with the outer surfaces of the pressure members 61, it is difficult to obtain sufficient engaging strength. As a result, as mentioned about the power steering apparatus, it is difficult to apply this structure to usage in which the transmission of a large force between the rotating cylinder 6 and moving shaft 7 is required.
As disclosed in Japanese Patent Application Laid-Open No. 59-9351 (1984), the reinforcement of the engagement can be improved by forming a thread groove on the outer circumferential surface of the moving shaft. However, even in the structure disclosed in Japanese Patent Application Laid-Open No. 59-9351 (1984), it is required for obtaining the suitable engagement condition between the feed rings and moving shaft to improve accuracy in processing and assembling of respective members. As a result, a large number of processing steps including adjustment of a dimension of a spacer coming into contact with one side of the feed ring are required.
Furthermore, in the case where a power steering apparatus is constituted by using the above-mentioned movement transforming device as a main component, an external force, such as a reaction force from tire wheels for steering connected with both ends of a steering shaft as the moving shaft, is applied to the steering shaft, and radial load to various directions acts. However, in the movement transforming device having the above-mentioned structure, the moving shaft is supported in the radial direction only by the plurality of feed rings along a lengthwise range of the rotating cylinder. However, these feed rings are engaged with the circumferential surface of the moving shaft at one portion in the circumferential direction, and the feed rings only can bear the radial load directing to the respective engagement positions. As a result, it is difficult to stably support the moving shaft on which the radial load directing to various directions is acting.
In order to solve these difficult problems in the conventional structure, three or more feed rings are used, and their engagement positions are set at substantially uniform intervals in the circumferencial direction so that the feed rings bear the radial load directing towards various directions by means of their synergistic action. However in this case, the moving shaft is supported by the feed rings at one point of the respective engagement positions, and thus in order to obtain desired capability of bearing the radial load, it is inevitable to enlarge the size of the feed rings. Moreover, this causes a problem such that the size of the rotating cylinder for holding the three or more feed rings with such a large size becomes larger, and thus it is difficult to miniaturize the movement transforming device having this structure equivalently to the ball-screw type movement transforming device.
The above-mentioned problem can be solved by proving a bearing for supporting the moving shaft in the radial direction to a housing inner diameter portion in the outside of the end portion of the rotating cylinder. However, the thread groove for reinforcing the engagement with the feed rings is formed on the outer circumferential surface of the moving shaft as mentioned above, and this thread groove slidably contacts with the bearing provided as mentioned above according to the movement of the moving shaft, and eccentric abrasion is generated on the bearing so that the satisfactory supporting state cannot be held. Moreover, the shape of the thread groove is changed by the sliding contact with the bearing, and thus there arises a problem such that the engagement condition of the feed rings is deteriorated.